set MACOSX_DEPLOYMENT_TARGET to whatever detected during translation

disutils yields an error if you set it to something other than that
configured, and I tested that error

--HG--
branch : macosx-deployment-target-2.7
1 job for branch/macosx-deployment-target-2.7 in 7 minutes and 18 seconds (queued for 2 seconds)
latest
Status Name Job ID Coverage
  Test
passed check_rpython_annotations #109251

00:07:18